1. Field of the Invention
This invention relates to a device for driving an electro-optical display element such as electrochromic display element (hereinafter simply referred to as the EC element) having a memorizing property.
2. Description of the Prior Art
The EC element is colored when the quantity of stored charge therein produced by the application of a first polarity, e.g., positive, voltage to the element exceeds a predetermined value, and the color is removed when the quantity of charge is decreased by application of a reverse polarity voltage to the element. This element has a property that once it has assumed a coloration state, it remains colored for a period of time even if the positive voltage applied thereto is cut off, as long as a reverse polarity, i.e., negative, voltage is not applied; namely, it has a memorizing property.
In a prior device for driving a display element such as EC element, when a display command signal is applied as an input, a color producing drive circuit is operated for a predetermined time to apply a positive voltage to the display element to thereby cause the display element to produce color and after the application of such voltage has caused, the color producing drive circuit is intermittently operated at predetermined intervals as long as the display command signal is applied in order to prevent the terminal voltage of the display element from being gradually decreased due to leakage of the stored charge, thus causing the display element to lose color. This device has the merit of saving electric power because the color producing drive circuit is intermittently operated. However, due to differences in pattern areas of the display elements, or the irregularity of the characteristic of the display element itself or the variation with time in the characteristic of the display element, the time required for the display element to change from its coloration state to its colorless state differs and varies. Accordingly, if the time interval at which the drive circuit is intermittently operated is predetermined and fixed, as in the prior device, there occurs an inconvenience in that the colorless state may occur when the coloration state should exist upon the application of a display command signal, or that a voltage is wastefully applied to the EC element even when the display element is already in the coloration state.